Chemical-mechanical planarization ("CMP") processes are frequently used to planarize dielectric layers in the production of ultra-high density integrated circuits. In typical CMP processes, a wafer is pressed against a slurry on a polishing pad under controlled chemical, pressure, velocity, and temperature conditions. Slurry solutions generally contain small, abrasive particles of silica or alumina that mechanically remove the surface of the wafer, and chemicals that react with the materials of the dielectric layers to enhance the removal of the molecules on the surface of the wafer. The polishing pad is generally a planar pad made from a relatively soft, porous material such as blown polyurethane.
CMP processes must accurately planarize the dielectric layer to a desired endpoint. Several hunched microelectronic devices are typically fabricated on a single wafer by depositing layers of various materials on the wafer, and manipulating the wafer and other layers of material with photolithographic, etching, and doping processes. In order to manufacture ultra-high density integrated circuits, CMP processes must provide a highly planar surface so that the geometries of the component parts of a die may be accurately positioned across the full surface of the wafer. Thus, it is important to accurately planarize the wafers to a desired endpoint across the whole wafer.
In the competitive semiconductor industry, it is also highly desirable to maximize the throughput of the CMP processes to produce accurate, planar surfaces as quickly as possible. The throughput of CMP processes is a function of several factors including the rate at which the thickness of the wafer decreases as it is being planarized (the "polishing rate"), and the ability to accurately stop the CMP process at a desired endpoint. A high polishing rate generally results in a greater throughput because it requires less time to planarize a wafer. Accurately stopping the CMP process at a desired endpoint is also important to maintaining a high throughput because the thickness of the dielectric layer must be within an acceptable range; if the thickness of the dielectric layer is not within its acceptable range, the wafer must be re-planarized until it reaches a desired endpoint. Such re-planarization of a wafer significantly reduces the throughput of current CMP processes, so it is highly desirable to stop the CMP process at a desired endpoint on the first attempt.
One problem with current CMP processes is that it is difficult to accurately stop the CMP process at a desired endpoint. Current CMP processes predict when the dielectric layer has been planarized to its desired endpoint by estimating the planarization time required to remove the desired amount of material. The planarization time itself is also estimated by using the average polishing rate of another wafer that was polished sometime before the contemporaneous wafer. The polishing rate, however, is a function of the several operating parameters including: (1) the downward pressure of the wafer against the slurry and pad; (2) the relative velocity between the wafer and the pad; (3) the chemical and abrasive characteristics of the slurry; (4) the condition of the pad; (5) the temperature of the slurry; and (6) the type of planarizing tool (e.g., single head or double head). Some of the operating parameters may change from one wafer to another in a controlled manner to improve the planarity of the dielectric layer or the throughput of the process. Other operating parameters may change in an uncontrolled manner. The polishing rate will accordingly vary from one wafer to another resulting in an inaccurate estimate of the planarization time and stopping point for the contemporaneous wafer. Therefore, it would be desirable to develop a method that provides better control of the polishing rate of a wafer to more accurately estimate the planarization time for a contemporaneous wafer.
Another problem with current CMP processes is that polishing rates change for a number of reasons, and it is difficult to determine which one of the variables must be corrected to bring the polishing rate back to a desired level. The downward pressure of the wafer and the relative velocity between the wafer and the pad are relatively easy to measure and indicate to the operator. The characteristics of the slurry, the temperature of the wafer, and the polishing condition of the pad, however, are difficult to ascertain. For example, polishing pads degrade over time and become less effective after planarizing a wafer because materials from the wafer and the slurry fill the pores and grooves on the surface of the pads and reduce the pads' ability to abrade subsequent wafers. Polishing pads are consequently "conditioned" to bring them back to an acceptable state for planarizing a wafer by abrading the built-up materials on their surfaces with a diamond-embedded stone. Accordingly, if the polishing rate drops for a reason other than the pressure or velocity, the operator must guess whether the drop was caused by the condition of the pad, the effectiveness of the slurry, or the temperature of the wafer. Therefore, it would be desirable to develop a method that generally eliminates the condition of the pad as a factor which would cause a change in the polishing rate of the pad.
One way to provide a better estimate of the planarizing time is to condition the pad after or while each wafer is polished. U.S. Pat. No. 5,216,843 discloses a pad conditioning process that better estimates the planarization time of pads that have concentric grooves on their planarizing surfaces. Such a pad conditioning process scores small, spaced-apart radial grooves in the pad and any material that builds-up on the surface of the polishing pad while a wafer is being polished. This pad conditioning process avoids changing the thickness of its pad to prevent mining the concentric grooves. The grooved pad conditioning process of U.S. Pat. No. 5,216,843, however, is not useful on flat pads because it requires large, permanent grooves in the pad to facilitate slurry transport. Moreover, without the large, permanent grooves, the grooved pad conditioning process will produce uncontrollable polishing rates in flat pads. Therefore, it would be desirable to develop a method for selectively reconditioning a flat pad after each wafer to provide a desired polishing rate.